It is desirable when treating cancers with radiation to have a high ratio of energy deposited in the tumor, relative to energy deposited in normal tissues surrounding the tumor, resulting in a high therapeutic ratio of tumor to normal dose. Radiation treatments using high energy electron or particle beams and high energy photon beams are used in the treatment of some cancers because these provide the higher tumor to normal surrounding tissue dose ratio with deeper penetration, relative to lower energy or x-ray beams. Such beams are typically provided by a linear accelerator, by a cyclotron, or related apparatus.
Charged particles, such us electrons, positrons, protons, or alpha particles, moving at greater than the effective speed of light in a medium tend to slow down while releasing Cherenkov radiation. Mammalian tissue, including human tissue, is a medium where the speed of light is reduced relative to air or vacuum due to its refractive index being greater than unity. Therefore fast-moving charged particles release Cherenkov radiation after entering such tissue. Water is also a medium where the speed of light is reduced relative to air or vacuum, fast-moving charged particles in water also release Cherenkov radiation after entering such water. Cherenkov emission has been detected with incident radiation in the range of 6 to 24 MeV energies for electrons, and for gamma-ray photons of 6 to 18 MV which Compton scatter to produce energetic electrons which in turn produce Cherenkov radiation. Since the threshold for emitting Cherenkov depends on velocity, and energy depends on both velocity and mass, Cherenkov radiation will be released from beams of protons and other charged particles at significantly higher energies.
When this Cherenkov light is induced in tissue, it is predominantly blue in color, but with a broad spectrum which tapers off into the green, red, and NIR with an inverse square wavelength dependence given by the Frank-Tamm formula. This light emitted in tissue is attenuated by absorbers in the tissue, and can also excite other molecular species in tissue, inducing their photo-luminescence (fluorescence or phosphorescence).
Prior to treating patients with particle beams, it is desirable to know the shape of the beam, and to verify that the beam shape is as planned. Additionally, when beams enter tissue it is important to accurately predict how radiation beam shape varies with depth in tissue, to ensure adequate dosage to tumor while minimizing dosage to surrounding normal tissues. If beam shape and position is adjusted by positioning deflection magnets or shielding devices, it can be important to confirm that the resulting beam shape and dosage profile are as desired prior to exposing patients to the beam; radiation treatment centers may therefore desire to confirm beam shape and dose profile for complex beam shaping procedures for each patient, or as part of routine calibration and maintenance.
Manufacturers of radiation treatment devices often prepare documentation of beam shapes and dosage profiles produced by common configurations of their devices for training users and guiding operators in using their machines to treat patients. Further, they must seek regulatory approvals of their machines, and as part of the regulatory approvals process they are expected to provide documentation of beam shapes and dosage profiles achievable by their machines. Manufacturers may therefore need to accurately verify and document beam profiles for this regulatory approval process.